Stories From the Buffer Zone

by Larry Kinney and Michael Stiles

Larry Kinney is president and Michael Stiles is vice president of Synertech Systems Corporation, an energy research and services organization in Syracuse, New York.

Basement Ducts

Oak Ridge National Laboratory's national evaluation of the Weatherization Assistance Program is nearing completion (see Weatherization Assistance: The Single-Family Study,). Part of this evaluation is a multi-phase Single-Family Study. In Phase I, information on retrofits completed in 1989 was gathered from local weatherization agencies and matched with utility consumption records, yielding almost 8,600 records from which energy savings were calculated.

To determine why certain dwellings saved more energy than others, nearly 800 dwellings in 20 states were inspected in Phase 2 of the Single-Family Study (600 weatherized, the rest control houses). Inspections yielded detailed descriptions of the dwellings, furnace efficiencies and safety inspections, blower door tests, and client interviews.

As an adjunct project, the Electric Power Research Institute (EPRI) gave us funding to test ducts in several hundred dwellings, focusing on structures with furnaces in basements. Besides documenting the condition of the ducts, a primary aim was to develop a series of duct leakage measurement protocols and assess their usefulness (see Multizone Rules of Thumb). We also ran a variety of tests on a closely monitored unoccupied test home, configuring ducts differently between sets of tests. Finally, we tested 30 houses with basements in New York and Pennsylvania.

In our field work, we used a blower door, the HVAC's own air handler fan, and a Duct Blaster. We sometimes measured pressure gradients with Magnehelic gauges (which need leveling), but more often (and more accurately) we used digital micromanometers.

Under various conditions, we measured flows using calibrated orifices (such as blower doors), velocity sensors, modified duct pressure pans, and flow hoods. House configurations were varied for specific tests. For instance, we opened and closed access doors to the furnace zone and from the furnace zone to the outside, and temporarily sealed all or a measured portion of the return and supply ducts.

Which tests to run under what circumstances is a decision of considerable consequence. To find leaks efficiently, seal them quickly, and verify that they are sealed, simple procedures with a blower door, a pressure pan, and some artificial smoke are frequently adequate. If the aim is to test the potential for backdrafting by determining the relative contributions of the HVAC and ventilation fans to negative pressure in the furnace zone relative to the outside, a set of tests with $50 worth of instruments that take less than ten minutes to perform is adequate. However, to quantify leakage from the supply and return sides, both to the furnace zone and to the outside of the envelope, more detailed protocols are necessary. Some multizone dwellings and duct systems are so complex they defy quantitative analysis altogether!

Have Protocols, Will Test

Our aim was to develop and test duct protocols to determine their usefulness under a variety of circumstances. Acting on an idea from Michael Blasnik of GRASP, we developed a format--for setting up a dwelling to be tested and for taking various measurements--that lent itself to simple steps between data collection sequences. Synertech researcher Tom Wilson designed a manifold for testing pressure differences between various areas in and out of the envelope and distribution system. This enabled us to gather data accurately and efficiently (see Multizone Rule of Thumb).

From 769 dwellings observed in 20 states, 379 dwellings with furnaces yielded sufficiently reliable data to infer the following: There were 296 furnaces with supply and return ducts. The length of their supply runs averaged 86 ft and their return runs averaged 27 ft. Data on another 68 dwellings was obscure concerning returns (no reliable data was collected), but had supplies which averaged 54 ft in total length. Fifteen dwellings (4%) had no return ducts at all, a dangerous situation that could cause backdrafting of the furnaces themselves or adjacent hot water heaters.

In basement structures, 76% of ducts are leaky according to the more than 100 cfm at 50 pascals criterion of the blower door subtraction test (see Leak Detectors: Experts Explain the Techniques, p. 26, and Two Favorite Test Methods, By the Book, p. 32). In spite of the fact that the return ducts are usually shorter, they leak more. Supply ducts are three times longer than return ducts, but net return leaks were longer than supply leaks in 60% of the cases we examined across the country, which included structures with and without basements.

In one of the blower door subtraction tests that we ran, the duct openings are temporarily sealed and the basement door to the inside is open. This effectively creates two zones, with the basement and dwelling constituting one zone and the ducts the second. If the ducts leak substantially to the outside of the envelope, pressures between the inside of the envelope and the ducts will be above 5 Pa. This occurred in only 5% of the cases measured. Hence, most leakage tends to be to the basement.

What are the consequences of this finding? When the basement is tight and the leakage is primarily on the return side, the HVAC system's distribution fan depressurizes the basement. Depending on its extent, this may or may not be a cause for alarm, but it means return air temperatures and system efficiency are lowered to the degree that the basement temperature is lower than that of the interior of house. Of course, supply leaks in the basement heat it unduly, thereby causing at least some conductive losses. At any rate, our knowledge of the real energy consequences of basement leakage (plus conductive and radiative components) is still quite sketchy.

New Construction Surprises

In a related project supported by the New York State Energy Research and Development Authority, we tested 50 homes in upstate New York (constructed between 1980 and the present) for relative tightness, using blower doors in both pressurization and depressurization modes. Forty-one dwellings had central heating units in their basements, 25 of which were forced air systems. In addition to blower door testing, pressures between the basement zone and the outside of the house, on one hand, and the main envelope and outside, on the other hand, were measured while

* Opening and closing the basement door.

* Turning ventilation fans (including the dryer) on and off.

* Turning the air handler fan on and off.

These combinations produced eight configurations. Worst-case pressure readings between the basement and outside usually occurred with the basement door closed and all fans on. In 20 of 25 cases involving forced air systems, return air leaks were greater than supply leaks, resulting in an average of -2 Pa and one case of -8 Pa due to the HVAC system alone! Many (by no means all) of the tighter houses are associated with high worst-case pressure measurements. Fortunately all of the dwellings which had substantial worst-case basement depressurizations also had power vented appliances since exhaust spillage from furnaces and (especially) hot water heaters can occur at only a few Pa. n

Blockages can occur anywhere in the system, including places that are difficult or impossible to reach. Leaks can occur between the ducts and the basement, between the ducts and the outside of the house, or between the ducts and the outside by way of the unconditioned space of the basement. This is further complicated by the fact that the basement typically leaks to the inside of the house and to the outside--which can befuddle users of many duct leakage test methods.

When testing ducts in complicated situations, a few rules of thumb help simplify and clarify the issues.

One must decide what needs to be learned about a distribution system, and decide how much effort and time to put into the tests. Is the goal to learn about problems with one branch of the system, or to learn about the whole system? Is the primary concern duct leakage in the return side or the supply side? Or is the issue whether the ducts leak to the outside? What kind of equipment is available? Is it better to use the HVAC system's own air handler to perform the tests? Is the goal to draw a quick and dirty conclusion about whether a set of ducts are tight or leaky, or is it to estimate how much of the conditioned air flow is wasted? Simply defining what one wants to do is the best way to select from among the many techniques for diagnosing duct leaks.

Whenever possible, combine the results of two or more tests. This also helps verify that results are accurate. Combining tests can also help identify the nature and location of a bad problem quickly. When a two-zone configuration can be set up, pressures between zones are measured under one condition and then another condition is imposed by opening or closing a door, by sealing outlets, or adding a hole of known size and flow characteristics to the interface between adjacent zones (see In Search of the Missing Leak, HE Nov/Dec '92, p. 27). The values of the pressure differentials measured under complementary conditions can be used to solve simultaneous equations that model the leakage pathways.

In Table 1, two typical duct testing methods are used. The first involves sealing all the registers and then measuring the pressure between each outlet and the house with the air handler running. The second method requires measuring the flow through each outlet when all the registers are open and the air handler is running. This second test is typically performed to see whether the system is balanced. But combining the results of the two tests can be very informative under certain conditions.

For example, if running the pressure test by itself indicates that there is a high pressure between a sealed supply register and the house, one may assume that there are no problems. However, it is possible for a restriction to be present in the line without affecting the pressures developed under the test conditions. It is only by finding a low flow rate at the register in the second test that the presence of a restriction can be surmised. A typical situation arises when in-line dampers are unintentionally closed. Dampers can be put in some very out-of-the-way places and can be missed during a visual inspection. A combination of duct tests can prompt one to go back and seek either the damper or the foreign object responsible for the restriction.

If the ducts are in multiple zones, it is best to remove a zone or two when performing leakage tests. This is already recommended for certain tests. During a blower door subtraction test, the difference in flow readings--when the registers are all open to the inside and when the registers are all taped shut--is taken as one measure of duct leakage (see Leak Detectors: Experts Explain the Techniques, p. 26). It is universally recommended that the basement be completely opened to the outside in order to provide a free and unrestricted flow of air to the duct leaks. In this case, the zone of the basement is removed by removing all pressure gradients between the basement and the outside.

Another variation of this theme is useful in testing for duct leaks to the outside of the house. This can be done quickly by sealing the registers and removing the pressure barriers between the house and the basement by opening a door between these two zones. With the blower door running, the ducts are then all in the same pressure zone with respect to the outside. The only source of leakage flow is from the outside of the house in this case, and the leaks to the outside are revealed if a pressure difference of more than 5 Pa develops between the house and the sealed ducts.

Special equipment can shorten time spent measuring pressure drops across multiple zones. This can be made from widely available materials. It's often necessary to run a tube between a zone and the place where a pressure gauge is being used to take readings to estimate leakage. We've developed a switching manifold for the tubes run to various parts of a house when measuring inter-zone pressures.

Suppose one wants to see how leaky the basement is to the outside word and to the inside of the house. And in the same house, one also wants to see how leaky the ducts are to the outside of the house and to the basement. It is advantageous to run the series of tubes from each of those locations back to a central location, such as near the blower door. The way to handle all these tests is to connect the tubes to a pressure gauge by way of a system of valves, so that one can easily switch the ports of the gauge from basement-to-outside, basement-to-inside, ducts-to-basement, and so on.

The idea behind using a pressure manifold may sound confusing at first, but with some field practice, almost anyone can design and fabricate one. It shortens field time and helps to keep track of what one is doing. When combined with an appropriate data-entry form, many results can be obtained in minutes once the hardware is in place.

Variety is the spice of multizone testing. We typically begin by preparing a dwelling for blower door testing. In addition, we run polyethylene tubes from a central location (usually close to the blower door) where they connect a manifold and manometer to such places as the basement, attic, return ducts, supply ducts, and the outside of the dwelling. We run an extension cord to the air handler so we can switch it on and off from a manifold and manometer. We then run a series of tests in an orderly sequence, measuring six to eight pressure differences under each configuration.

In the interests of efficiency, we usually vary only one or two parameters between tests. For example, a first blower door test is run with the basement door open, a second with it shut, a third test with ducts sealed and the outside basement door open, and so on. Then we run a series of tests with the blower door off and sealed, but with the air handler on. We test pressures with no ducts sealed, then with supply sealed only, then supply and return, then return with a hole of known size added.

Data are keyed into a laptop computer in real time. A spreadsheet tracks the results of each test so that field technicians can compare results and catch inconsistencies while the equipment is still set up. (It's also nice to have the data tabulated while still in the field. Reports are 90% complete by the time we leave a house.) Finally, the results of these tests (and other tests of flow and pressure differences at each duct) are used to direct attention to holes, blockages, and so forth. These are inspected, measured and photographed.

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